Screw-thread cutting die



A- H. LLOYD SCREW THREAD CUTTING DIE Filed Oct. 18. 1929 [772/672 for:

Patented July 7, 1931 UNITED STATES ARTHUR HAROLD LLOYD, OF COVENTRY,ENGLAND SCREVf-THREAD CUTTING DIE Application filed October 18, 1929,Serial No.

This invention relates to dies for cutting screw threads, of the kindhaving segments with a leading cutting edge and a guiding portion whichengages the newly cut thread and extends tangentially thereof (orapproximately so) both in the leading and the following direction of thecutting movement.

The primary object of the invention is to provide means for rectifyingthe die segments by means of a lapping hob, in such a way as toeliminate any distortion due to hardening and to ensure that the correcthelix angle of the die threads is maintained after sharpening so that agood finish is obtained at all times.

This and further objects and advantages of the invention will beapparent from the following description, in which reference is made tothe accompanying drawings, wherein Figure 1 is a plan of a die segmentengaged with a three-start-thread lapping hob adapted according to theinvention,

Figure 2 shows a portion of a screw-thread out by the die segment,

Figure 3 is a front view of the die seg ment,

Figure 4 is a diagrammatic representation showing an end view of the diesegment cut ting a screw thread (though the depth of the thread isactually ignored for the sake of simplicity, the thread being regardedas having no depth),

Figure 5 is a diagrammatic front view of the die indicating the area ofcontact between the die and the lapping hob, and

Figure 6 is a plan of part of a two-startthread lapping hob with one ofthe startthreads missing.

In the construction described in the specification of British Patent No.8806/04, die

segments are provided with straight V grooves extending tangentially tothe sides of the threads with which they are adapted to engage forguiding purposes. It will be understood, however, that the guidingefiect obtained thereby is merely by a line contact at each threadbecause the latter is of helical form, whilst the groove sides arestraight.

400,614, and. in Great Britain July 9, 1929.

Thus, inaccuracy in the control of the thread formation can readilyarise.

To overcome this difficulty, it has been proposed to use a lapping hob,having a thread corresponding to that of the screw to be cut, which hobis engaged with the previouslyhardened die-grooves substantially as thework would engage with the latter, and is then charged with abrasive andoscillated about its axis (or rotated in opposite directionsalternately) whilst the die segments are resiliently, or otherwise, fedinwards. This corrects the sides of the grooves so that they conform tothe sides of the finished thread over considerable arcs and it providesaccuracy of pitch and rectifies any distortion due to hardening.However, to avoid too close a contact of the die with the work, the hobhad to be made of somewhat greater diameter than the screw to beproduced, and consequently with a thread of smaller helix angle, whichhad the effect of producing a rough finish on the work.

In the attempt to overcome this difficulty, it has been proposed, duringthe lapping process, to incline the die segments to the axis of theoversize hob, but in such a case the die is only correctly disposed tothe work in one position of its cutting edge, and when new. Whenre-sharpened an error is introduced which causes rough work.

In accordance with the present invention, a multi-start threaded lappinghob is-used to correct the die, the effective diameter of the hob beinga multiple of the effective diameter of the screw to be produced and thenumber of starts being equal to that multiple. In this the helix angleof the hob can be the same as that of the screw to be produced but itsdiameter is considerably larger.

In one example, die segments to cut a screw with 11 Whitworththreads tothe inch would be lapped with a hob having 11 threads to the inch and,say, two starts and of an effective diameter twice the effectivediameter of the screw. Or, a hobwith an effective diameter three timesthe effective diameter of the screw and with three starts could be used,and so forth, the multiples of the effective diameter of the screw beingthe same as the number of starts.

To arrive at the effective diameter, the depth of the thread must betaken into consideration. For example, in the case of a lVhitworthscrew, the outside diameter is .625 and the depth of the thread .058",making the effective diameter .567. The effective diameter of atwo-start hob is twice this figure, i. e. 1.134, to which must be addedthe depth of the thread, making the overall diameter of the hob 1.192.

In the arrangement shown by Figures 1-5 of the drawings, where 2represents the die segment, 3 the screw and 4 the lapping hob, thelapping hob is shown provided with a three-start thread, the respectivestarts being indicated by a dotted line 5, a broken line 6 and a fullline 7 Assuming that there are in this case eleven threads to the inchon the screw and eleven grooves to the inch on the die segment, thereare then 11/3 threads to the inch in each start on the lapping hob.

In general, if there are m threads per unit length of the screw, thereare m/n threads per unit length of each start, where n is the number ofstarts on the hob.

Ifd is the effective diameter of the screw 3 (being equal to the overalldiameter less the depth of the thread) and D the effective diameter ofthe hob, then, in accordance with the present invention,

or, in the case of the three-start thread shown in Figure 1, theeffective. diameter of the hob is three times that of the screw.

It will be realized from a comparison of Figures 1 and 2 that with thisarrangement the helical angle of the screw is the same as the helicalangle of the lapping hob.

In employing the hob 4, the die segments are arranged radially and withtheir grooves 20 in engagement with the threads on the hob, as indicatedby Figure 1, and the hob is then rotated or oscillated whilst coatedwith abrasive in the well-known manner.

The threaded face of each die segment is usually milled at an angle toits flanks, and the segments must be so arranged with regard to the axisof the hob that a plane passing through the axis of the hob, and theultimate axis of the screw to be produced, is at right angles to theeffective plane in which the threaded grooves lie. As the lapping iseffected without any inclination of the axis of the hob, the segment isuniform and correct through the whole of its life.

Figure 4 shows the effective plane 8, in which the groves 20 of the diesegment lie, disposed at right angles to the chain line 9 whichrepresents a plane passingthrough the axis of the hob (shown partly inchain line at 4) and the axis of the screw 3- to be produced. Theeffective plane 8 is slightly lapped away at the zone of contact 10between the hob and the die; this zone of contact, for clearness, isshown in exaggerated form in Figures 4 and 5, the arrow 11 indicatingthe junction of the rectified portion with plane 8. In Figure 3, 12indicates the rectified or threaded parts of the V grooves 20 of the diesegment, the same being exaggerated for clearness.

It is obvious that in some cases one or more of the multi-threads 5, 6or 7 can be removed and the hob 4 fed along axially to ensure that allthe grooves 20 of the die will be rectified.

Figure 6 shows a two-start thread on a hob of efiective diameter twicethat of the screw to be cut, but one start-thread, indicated by thechain lines 13, has been removed. By rotating the hob with its remainingstartthread engaging the V grooves, the hob will travel axially withrespect to the die segment and thus rectify all the groves 20.

It is to be understood that slight variations from the foregoingdimensions may be adopted, for manufacturing reasons, without departingfrom the spirit of the invene tion.

What I claim as my invention and desire to secure by Letters Patent ofthe United States is 1. A method of rectifying, by the use of a lappinghob, a die segment for cutting a screw of efiective diameter d with mthreads per unit length, which involves the use of a lapping hob of aneffective diameter of n x d where n is an integer greater than 1) andwith m/n threads per unit length, substantially as described. V

2. A method of rectifying, by the use of a lapping hob, a die segmentfor cuttinga screw-thread, which involves the use of amulti-start-threaded lapping hob having an effective diameter the ratioof which to that of the screw to be cut is equal to the number ofstarts, substantially as described.

3. A method of rectifying a die segment for cutting a screw thread, bythe use of a lapping hob having a number of starts of' the thread equalto the ratio of the effective diameter of the hob to the effectivediameter of the screw.

4. A method of rectifying a die segment for cutting a screw thread, bythe use of' a multi-start-threaded lapping hob having. an effectivediameter greater than the effective diameter of the screw and .alsohaving the number of starts of the thread equal to the ratio of theeffective diameter of the hob to the effective diameter of the screw.

In testimony whereof I affix my signature.

ARTHUR HAROLD LLOYD,

